tube amp question

Someone I know wants to sell me a tube amp. I know very little to nothing about tubes, since I haven't played through one. He says the problem with it is that you play it for a little while and it sounds great. After 15-20 minutes, it sounds like crap. After letting it rest, it will do the same thing again, good for a little bit, then crap again. Anyone have any ideas? Is it an easy fix? Does it just need new tubes? Thanks.

Hard to diagnose on this limited info. Maybe it's a tube, but to be truthful, that really doesn't seem like the way a tube normally fails. I suspect another component is drifting off as the amp warms up.

The good news, most likely this is a cheap resistor or capacitor going south. The bad news, it could be hard to find, even a good amp tech is going to take a while to trace the problem.

A good SVT-CL sells for something like 1500 new, IIRC. So used-in decent shape- would be half of that. But people seem to ask more on Ebay.

An amp (SVT-CL) with unknown problems (and an unknown history of possible abuse), I wouldn't give more than 500. And that's considering I would be repairing it myself instead of paying for a repair.

It's probably a filter cap. Those things typically go bad after a few years. As I recall that amp has a few big caps in it, those may be hard to find in terms of an "original" replacement, but I'm sure you could find something that will work, on mouser.com or one of the online parts supply places.

nashvillebill is right, it's hard to diagnose, it could be almost anything causing that symptom. Definitely check the tubes. As far as value, a new set of tubes for that amp will run you a couple hundred bucks (if you get decent ones), and if you have a tech do a cap job for you that'll be another hundred fifty or two hundred (depending on how hard it is to get the parts and how expensive they are), so figure you'll have to invest three or four hundred extra dollars into it, on top of whatever you're paying the seller.

I would suggest, before throwing in all new tubes, have it checked thouroughly first by a good amp tech. It could be a cap, or some other type of problem. May not be the tubes at all. If it turned out to be tubes, something else could be happening to make them go bad in the first place.

BUSTED!!!
Are you in it or just spectating?...Or is that classified top secret information? It looks super tempting, but ebay is always a gamble. I'm seriously considering on going on an all-out quest for a PHAT tube amp, even though I'm broke.

I've got two SVTs already... so I was just watching out of curiousity's sake. But I am always on the lookout for a good deal on my favorite kind of amp (all tube). Had to let that one go - even though it was probably a great deal. What did it go for??